1. Field of the Invention
The present invention relates to an image display apparatus and image display method for displaying two images of the same subject obtained, for example, at different times, and a storage medium storing a program for implementing the image display method.
2. Prior Art
Medical X-ray images, in particular X-ray images of the human chest, are indispensable to medical diagnosis, and are very widely used even today.
Chest X-ray images are used for examining the extent of progress of a disease or for examining for changes indicative of outbreak of a disease in a patient who is the subject of the X-ray radiography. Such examinations are often carried out by comparing current and past X-ray images of the same patient. In such a comparison, the past image and the current image are generally displayed simultaneously or films thereof are presented simultaneously, and an observer looks for differences between the two.
Moreover, as described in “Application of digital image change detection to diagnosis and follow-up of cancer involving the lungs; Kinsey, J. H. and Vannelli, B. D.; SPIE70: 99-112; 1975”, attempts are being made to develop a method in which a differential image between a past image and a current image is generated and this differential image is viewed, thus allowing the differences between the past image and the current image to be seen clearly. Research into making this method fit for practical use is still in progress.
A specific problem being researched is how to cope with changes in the radiographic conditions between the past image and the current image. In particular, the positional relationship between the X-ray source, the radiographic subject and the image receiving section may vary greatly due to differences in the radiographic environment and the radiographer between the past radiography session and the current radiography session.
As a result, the straightforward difference between the past image and the current image may indicate changes in the position from which the X-ray was taken rather than changes in the lung field as intended. A technique commonly used at present is called “image warping”, which involves paying attention to the shape of the ribs in particular in the past image or the current image and warping the image, thus allowing the difference between the two images to be obtained with improved accuracy (the shadow of the ribs is eliminated upon taking the difference) (see, for example “Digital chest radiography: Effect of temporal subtraction images on detection accuracy; Difazio, M. C., MacMahon, H. M., Xu X-W., et al.; Radiology 202: 447-452; 1997”).
Moreover, from the standpoint that eliminating the shadow of the ribs is different to observing changes in the shadow of the lung field, which is the original objective, an alternative method has been devised in which attention is not paid to the ribs, but rather only the lung markings are extracted from the images and the difference between the images is obtained by aligning the lung markings (see “Hai-monri o taishou to shita gazou-ichi-awase-hou—Kyoubu X-sen gazou no jikanteki-sabun-hou (Image alignment method targeting lung markings—Temporal difference method for chest X-ray images); Sanada et al.; Japanese Journal of Radiological Technology; Vol. 56, No. 3, 398-404; 2000”).
However, the radiographic subject is generally 3-dimensional having depth, but the radiographed image is only a 2-dimensional projection of the radiographic subject. Regardless of how this projected image which gives only 2-dimensional information is changed through warping, it is thus fundamentally impossible to obtain the difference between two images in 3 dimensions. Moreover, image warping generally requires complex image processing steps, and thus a great deal of computation.